This invention relates to the polymerization of macrocyclic polyester oligomer compositions. More particularly, it relates to an improved method for such polymerization which is capable of producing novel branched polyesters.
The preparation of macrocyclic poly(alkylene dicarboxylate) oligomers and their polymerization to linear polyesters is described in U.S. Pat. Nos. 5,039,783, 5,214,158 and 5,231,161 and in copending, commonly owned applications Ser. No. 08/369,986 now U.S. Pat. No. 5,466,744 and Ser. No. 07/978,583 now U.S. Pat. No. 5,321,117. The catalysts employed for such polymerization include various organotin compounds and titanate esters.
Polymerization using these catalysts is quite successful and affords polyesters having excellent properties and a wide variety of potential applications. However, the catalysts are somewhat sensitive to impurities present in the macrocyclic polyesters, particularly acidic impurities which can include water, hydroxy compounds and carboxylic acids and their anhydrides.
In the presence of such impurities, the catalyst may be partially deactivated and polymerization may be incomplete or may yield polymers with low weight average molecular weights. Attempts to increase polymer yield by increasing the proportion of catalyst in the polymerization mixture cause significant further reductions in the molecular weight of the linear polymer, since the catalyst becomes part of the polymer end group and increased amounts of catalyst compete for the same proportions of structural units in the macrocyclic oligomers.
In copending, commonly owned applications Ser. Nos. 08/262,793, 08/262,795 and 08/262,799 now U.S. Pat. Nos. 5,386,037; 5,389,719, and 5,387,666 respectively, there are disclosed several cyclic tin catalysts which offer improvements with respect to the above-described problems. However, these tin compounds are expensive to prepare and there are environmental and health concerns about their use,
It would be desirable, therefore, to develop safe and inexpensive catalysts with high activity for the polymerization of macrocyclic oligomers containing increased proportions of impurities, especially acidic impurities. It would be further desirable to provide a means for inexpensively converting macrocyclic oligomer compositions to branched polyesters having improved dimensional stability, for use as load-bearing members in automobiles and the like.